DE3687593T2 - METHOD FOR CARRYING OUT THERMAL CHOCKS IN THE ELECTRONICS AREA. - Google Patents

Abstract

Perfluoropolyethers having an average molecular weight higher than 390 and respectively having a kinematic viscosity lower than 8.5 cSt (at 20°C), and such as to distill by not more than 10% at temperatures lower than 140 °C, and by at least 90% at temperatures not higher than 260°C, or, in the case in which the perfluoropolyether does not contain CF-(CF₃)CF,O units, having a viscosity lower than 18 cSt (at 20°C), and such as to distill by not more than 10% at temperatures lower than 140 °C, and by at least 90% at temperatures not higher than 280°C, are used as the only high-temperature and low- temperature working fluid in the Thermal Shock Tests to which the electronic components are submitted, and at the same time are advantageously used in other tests used in the electronic industry, such as the Gross Leak Test and the Burn in Test, allowing the operators in this field to use one single fluid for a whole set of uses.

Description

Background of the invention 1. Field of the invention

The present invention relates to a method for carrying out thermal shock tests in the electronics industry.

In particular, the invention relates to a method for carrying out such tests using perfluoropolyethers.

2. Description of the state of the art

Thermal shock testing (TST), the details of which are described in US MIL STD 883-1105.1, involves subjecting electronic components to high and low temperature thermal cycles and then testing both the physical properties of the materials and their electrical performance.

In practice, the components are immersed alternately and repeatedly in a hot inert liquid and in a cold inert liquid. The temperatures at which the tests are usually carried out depend on the level of reliability required for the electronic components. The most suitable temperature pairs are: -55 and +125ºC; -65 and +150ºC; -65 and +200ºC; in these cases, temperature variations of +10ºC for the hot baths and -10ºC for the cold baths are allowed.

The transfer of the electronic devices from one bath to another and vice versa must take place within very short periods of time, not longer than 10 seconds.

Highly fluorinated liquids are traditionally used for this test. In fact, it is known that the compounds with high fluorine content have an exceptionally favorable combination of excellent properties, e.g. chemical inertness, thermal stability, non-flammability, high electrical resistance, low surface tension, poor water solubility, compatibility with many materials such as elastomers, plastomers and metals.

Perfluoroalkanes with linear or cyclic structure, obtained by fluorination of aliphatic, cycloaliphatic or aromatic hydrocarbons, are known. A representative example of cyclic compounds is perfluorodimethylcyclohexane, which is obtained by reacting xylene with CoF3.

However, the liquids obtained in this way are not completely fluorinated because they contain by-products that still contain hydrogen atoms. The presence of such by-products reduces the thermal stability and chemical inertness of such fluorinated liquids, which limits their field of application.

Furthermore, these liquids do not have very high boiling temperatures if their pour points are very low, even if they are fully fluorinated.

The term "pour point" is understood to mean the temperature at which the liquid modifies its physical properties after cooling, i.e. at which its fluidity decreases due to an increase in viscosity. In general, the pour point is considered to be the temperature at which the viscosity reaches 100,000 cSt (ASTM D 97 standard). For example, for pour points of -70ºC, the boiling point is at most of the order of 100ºC, while products with higher boiling points, of the order of 210ºC, suffer from the disadvantage of having pour points that are too high, of the order of -20ºC.

Perfluorinated compounds with ether or amine structure are also known, which are obtained by electrofluorination in hydrofluoric acid of the corresponding hydrogenated compound. A representative example of these compounds is perfluorotributylamine.

Also in this case, the liquids obtained are not completely fluorinated, as happens for the liquids described above, and present the same disadvantages.

Table 1 lists the physical properties of some of the products mentioned above. TABLE 1 Product Boiling point Pour point Perfluorotributylamine Perfluorotripentylamine Mixture of cyclic ethers of the formula C₈F₁₆O Perfluorodimethylcyclohexane

As can be seen, one group of these products can only be used at high temperatures, while another group can only be used at low temperatures. Indeed, the range for the boiling temperature and pour point is generally relatively wide, but not wide enough to allow the same perfluorinated compound to be used at both high and low temperatures.

The compounds belonging to the class of amines have high Pour points if they have relatively high boiling points; the class of cyclic ethers has a pour point that can also be very low, but the corresponding boiling temperatures are low.

From GB-A-1 104 482 and US-A-3 665 041 it was known that perfluoropolyether fractions are suitable as liquids for heat exchange and are heat-resistant.

Perfluorinated liquids have been considered in the prior art as ideal liquids for TST baths, as detailed in EVAL. ENG. (USA), Vol. 9, No. 6, 1970. On page 8, this article mentions the use of FC77 and FC43 as discrete perfluorinated liquids for the low and high temperatures in the TST.

The use of pairs of different fluids for the low and high temperatures in the TST involves a problem that is both practical and economic in nature.

In practice, the rapid transfer of the parts under test from the cold to the hot bath and vice versa, due to the dripping of the liquid, causes: (1) mutual contamination of the baths and consequent changes in the physico-chemical properties of the liquids; (2) loss by evaporation of aliquots, which may also be appreciable, of the low boiling liquid when the latter is dragged into the high temperature tank and when the hot parts are immersed in the cold bath; (3) increase in the viscosity of the low boiling liquid contaminated by the high boiling liquid; (4) simultaneous decrease in the level of the cold liquid and increase in the level of the hot liquid (in fact, it happens that more cold liquid is dragged in, which is very viscous under the conditions of use); (5) necessary shutdown of the equipment from time to time to evaporate the liquids (6) the need to have a rectification unit on hand to recover the two mutually contaminated liquids.

There is therefore a need for a method for performing thermal shock tests using a liquid which is individually suitable for use at both high and low temperatures in order to avoid the disadvantages described above.

The present invention

The aim of the present invention is therefore to provide a method for carrying out thermal shock tests using only a single liquid for both the low and high temperatures, which meets the requirements for chemical inertness and at the same time has a high boiling temperature, higher than the temperature at which the test is carried out, and good flowability at low temperatures, as indicated by the pour point.

The subject of the present invention is therefore a method for carrying out thermal shock tests, which comprises:

(A) immersing the sample in a liquid perfluoropolyether having a molecular weight of at least 390, at a temperature between -75 and -55ºC, and selected from perfluoropolyethers comprising structural units of the following type:

1) (CF(CF₃)CF₂O) and (CFXO) randomly distributed along the perfluoropolyether chain, where X = -F, -CF₃;

2) (CF(CF₃)CF₂O);

3) (CF(CF₃)CF₂O), this class further includes the characteristic group -CF(CF₃) -CF(CF₃)-;

4) (CF(CF₃)CF₂O), (C₂F₄O), (CFXO), statistically along the perfluoropolyether chain, where X is -F, -CF₃;

5) (C₂F₄O), (CF₂O), randomly distributed along the perfluoropolyether chain;

6) (CF₂CF₂CF₂O);

7) (C₂F₄O);

said perfluoropolyethers, when CF(CF₃)CF₂O units are present, exhibit a kinematic viscosity below 8.5 cSt at 20°C and a distillation loss under atmospheric pressure of not higher than 10% by weight at a temperature below 140°C and at least 90% at temperatures not higher than 260°C, while when CF(CF₃)CF₂O units are absent, they exhibit a kinematic viscosity below 18 cSt at 20°C and a distillation loss of not more than 10% at temperatures below 140°C and at least 90% at temperatures not higher than 280°C;

(B) removing the sample from the perfluoropolyether of operation (A) and immersing it in a second liquid consisting of the same perfluoropolyether as in operation (A) at a temperature between 125 and 210ºC.

Perfluoropolyethers with a viscosity below 6 cSt (at 20ºC) and a 10-90% distillation range that is between 150 and 230ºC, or with a viscosity below 10 cSt (at 20ºC) with a 10-90% distillation range that is between 150 and 250ºC for perfluoropolyethers that do not comprise CF(CF₃)CF₂O units are preferred.

The viscosity in the present invention is always measured at 20°C.

The perfluoropolyethers containing the indicated units are known and are preferably selected from the following classes:

where X = -F, -CF₃; A and A', equal or different from each other, can be -CF₃, -C₂F₅, -C₃F₇; the units CF(CF₃)CF₂O and CFXO are randomly distributed along the perfluoropolyether chain, m and n are integers, n can be 0 and the ratio min ≥ 2 if n ≤ 0 and is such that the viscosity is below the value of 8.5 cSt given above.

These perfluoropolyethers are obtained by the hexafluoropropene photooxidation reaction according to the process described in British Patent No. 1,104,482 and subsequent conversion of the end groups into chemically inert groups as described in British Patent No. 1,226,566;

wherein B may be -C₂F₅, -C₃F₇ and m is a positive integer and such that the viscosity of the product is below the value of 8.5 cSt given above. These compounds are prepared by ionic oligomerization of hexafluoropropene epoxide followed by treatment of the acyl fluoride (-COF) with fluorine according to the procedures described in U.S. Patent No. 3,242,218;

where m is a positive integer such that the viscosity of the product is below the above-mentioned value of 8.5 cSt.

These products are obtained by ionic telomerization of hexafluoropropene epoxide and subsequent photochemical dimerization of the acyl fluoride according to procedures described in U.S. Patent No. 3,214,478;

4) A'O[CF(CF₃CF₂O]m(C₂F₄O)n(CFXO)q-A, wherein A and A' may be the same or different from each other, -CF₃, -C₂F₅, -C₃F₇; X is -F, -CF₃; m, n and q are integers and may also be zero, but in any case are such that the average molecular weight is at least 390 and the viscosity is within the limit given above (8.5 cSt).

These products are prepared by photooxidation of mixtures of C₃F₆ and C₂F₄ and subsequent treatment with fluorine according to the procedure described in USP 3,665,041;

5) CF₃O(C₂F₄O)p(CF₂O)q-CF₃, where p and q are integers, equal to or different from each other, in which the ratio p/q is between 0.5 and 2 and such that the viscosity is within the limit indicated above (18 cSt). These perfluoropolyethers are prepared as described in USP 3,715,378 and then treated with fluorine according to USP 3,665,041;

6) AO-(CF₂CF₂CF₂O)m-A', wherein A and A', equal or different, can be -CF₃, -C₂F₅, -C₃F₇ and m is an integer such that the viscosity of the product is below the value of 18 cSt given above.

These products are obtained according to European patent application EP 148 482;

7) DO-(CF₂CF₂O)r-D', wherein D and D', equal or different, may be -CF₃₁ -C₂F₅ and r is an integer such that the viscosity of the product is below the value of 18 cSt given above.

These products are obtained according to USP 4 523 039.

The liquids according to the invention exhibit the property that they have a narrow molecular weight distribution, with both the highly volatile and the high-boiling fractions being absent.

In addition, such fluids, while being characterized by a low pour point, exhibit a sufficiently low viscosity for use even at the very low temperatures indicated.

Another property of the liquids according to the invention is their volatility; in particular in the case of a viscosity of less than 4 cSt, removal by evaporation from the components is easily obtained at the end of the test, so that the tested component can be used without having to subsequently wash off the residues of the liquid used in the test.

In case of viscosity above 4 cSt, washing with chlorofluorocarbon solvents, e.g. Algofrene 113®, is preferred to remove any residue of the test fluid from the component.

The simultaneous presence of these properties in a single liquid makes this class of products particularly suitable for use in TST.

The following examples are given for the purpose of illustrating the invention only and are not intended to limit it.

EXAMPLE 1

Two stainless steel tanks A and B with a capacity of 20 liters, the temperature of which can be set to temperatures higher (tank A) or lower (tank B) than room temperature, are prepared.

In the two tanks, 24.1 kg and 36.7 kg respectively of a mixture of perfluoropolyethers of the general formula:

which, based on 19F NMR data, has a ratio min = 3.8 and A = A' = CF3, with minimal amounts of C3F7 and C2F5 end groups present.

The average molecular weight, determined by the VPO method (vapor pressure osmometry), is 480.

The perfluoropolyether has a kinematic viscosity η = 2.2 cSt (at 20ºC), a distillation range according to the method of ASTM 1078 of 161ºC-211ºC, a pour point = -90ºC and a specific density of 1.79 g/ml at 20ºC.

For tank A, a temperature of +125ºC is selected, reached within 25 minutes, and for tank B, a temperature of -55ºC, reached within 55 minutes, is selected.

Using a basket, a set of chip carriers with plastic containers and a set of resistors coated with ceramic material are alternatively inserted into the two tanks.

The entire cycle lasts 60 seconds, with the total residence time within each tank plus the transfer time being 30 seconds.

After 100 cycles, the two tanks contain 26.6 kg and 32.8 kg of fluid respectively.

Most of the lost liquid is found on the work table between the two tanks; 0.6 kg of it is recovered, so that the total evaporation loss is not higher than 1.3 wt%.

At the end of the tests, the fluid appears unchanged and no residues are found either in the fluid or in the heat exchange coils or anywhere else.

EXAMPLE 2

The test is carried out as described in Example 1 and using the same liquid, with the difference that the residence time of the devices being tested inside the cold tank is increased to 90 seconds and the number of cycles is reduced to 20.

It is observed that the temperature fluctuations in the hot tank reach the value of 1ºC and in the cold tank 8ºC.

Similar to the observations in Example 1, neither the formation of residues nor changes in the liquid are observed.

EXAMPLE 3

The test is carried out as described in Example 1, using a liquid with the same general formula as described above in point (1) and with the same m/n ratio, but characterized by an average molecular weight of 490, viscosity η= 2.38 cSt (at 20ºC), 10-90% distillation range of 162-218ºC, specific gravity 1.80 g/ml, with the difference that the temperature of the liquid is set at -65 or +150ºC, and that the number of cycles becomes 25 and the residence time 90 seconds or 1 hour.

30 seconds.

It is observed that the temperature of the cold bath fluctuates between -65ºC and -74.8ºC, and that the perfluoropolyether remains liquid enough not to cause any problems. During the test, the liquid level in tank A rises by 2 cm due to the effect of cold liquid entrainment by the basket containing the chip carriers. No significant evaporative losses of liquid from the high temperature tank are observed.

EXAMPLE 4

Using the same system of tanks as described in Example 1, with the difference that they are connected by a level line so that the liquid can move freely between the two tanks and that the two tanks are brought to constant temperatures of -65ºC and +150ºC respectively, 100 cycles with residence times of 30 seconds in each tank are carried out without inconvenience.

No temperature fluctuations are observed in the high temperature bath, while the temperature fluctuations in the cold bath are not higher than 5ºC. No noticeable liquid losses due to evaporation are observed.

EXAMPLE 5

A plant similar to that of Example 1 is used, but with 5-litre tanks, each equipped with two thermocouples for reading the temperature on two diametrically opposite sides and filled with the same polyether as in Example 3.

The tanks are brought to a constant temperature of +125ºC or -55ºC and two test series, each with 40 cycles, with residence times of 5 minutes in the high temperature bath and of 5 minutes in the cold bath are carried out under strict compliance with the working conditions as per standard MIL 883C, method 1011.5. The temperature fluctuations are of the order of about 3ºC for the cold bath and about 1ºC for the hot bath. In the hot bath the thermocouples never show mutual temperature differences of more than 0.1ºC and in the cold bath these differences never exceed 0.8ºC.

At the end of the test, it is observed that the level in the high temperature tank has increased from 9.9 to 10 cm (as measured at 125ºC), while in the other tank the level has fallen from 8.6 to 8 cm (as measured at -55ºC).

The increase in liquid volume in the hot tank is 50 ml and the liquid loss from the cold tank is 350 ml, most of which is collected on the work table between the two tanks.

Claims (7)

1. A method for carrying out thermal shock tests, which includes: (A) immersing the sample in a liquid perfluoropolyether having a molecular weight of at least 390, at a temperature between -75º and -55ºC, and selected from the perfluoropolyethers comprising structural units of the following type: 1) (CF(CF₃)CF₂O) and (CFXO) randomly distributed along the perfluoropolyether chain, where X is -CF₃; 2) (CF(CF₃)CF₂O); 3) (CF(CF₃)CF₂O), this class further includes the characteristic group -CF(CF₃)-CF(CF₃) 4) (CF(CF₃)CF₂O), (C₂F₄O), (CFXO) randomly distributed along the perfluoropolyether chain, where X is -F, -CF₃; 5) (C₂F₄O), (CF₂O) randomly distributed along the perfluoropolyether chain; 6) (CF₂CF₂CF₂O); 7) (C₂F₄O) said perfluoropolyethers, when CF(CF3)CF2O units are present, exhibit a kinematic viscosity below 8.5 cSt at 20°C and a distillation loss under atmospheric pressure of not higher than 10% by weight at a temperature below 140°C and at least 90% at temperatures not higher than 260°C, while when CF(CF3)CF2O units are absent, they exhibit a kinematic viscosity below 18 cSt at 20°C and a distillation loss not higher than 10% at temperatures below 140°C and at least 90% at temperatures not higher than 280°C;. (B) Taking the sample out of the perfluoropolyether of operation (A) and immersing it in a second liquid consisting of the same perfluoropolyether as in operation (A), at a temperature between 1250 and 210ºC. 2. A process according to claim 1, wherein the perfluoropolyether, when it comprises -CF(CF₃)CF₂O units, exhibits viscosities below 6 cSt at 20°C, the 10-90% distillation range being between 150° and 230°C, while when it does not comprise -CF(CF₃)CF₂O units, it exhibits viscosities below 10 cSt at 20°C, the 10-90% distillation range being between 150° and 250°C. 3. Process according to claims 1 and 2, wherein the perfluoropolyether is selected from the following classes : 1) wherein X is -F, -CF₃; A and A', equal or different from each other, can be -CF₃, -C₂F₅, -C₃F₇; the units CF(CF₃)CF₂O and CFXO are randomly distributed along the perfluoropolyether chain, m and n are integers, n can be zero and the ratio m/n is ≥ 2 when n ≤ 0 and is such that the viscosity is below the value indicated above; 2) wherein B can be -C₂F₅, -C₃F₇ and m is a positive integer and such that the viscosity of the product is below the value indicated above; 3) where m is a positive integer such that the viscosity of the product is below the value given above; 4) A'O[CF(CF₃)CF₂O]m(C₂F₄O)n(CFXO)q-A. wherein A and A', equal or different, may be -CF₃, -C₂F₅, -C₃F₇; X is -F, -CF₃; m, n and q are integers and may also be zero, but in any case are such that the average molecular weight is at least 390 and the viscosity is within the limit as indicated above; 5) CF₃O(C₂F₄O)p(CF₂O)q-CF₃, where p and q are integers equal to or different from each other, the ratio p/q is between 0.5 and 2, and is such that the viscosity is within the limit as indicated; 6) AO-(CF₂CF₂CF₂O)m-A', wherein A and A¹, equal or different, can be -CF₃, -C₂F₅₁ -C₃F₇ and m is an integer such that the viscosity of the product is below the value indicated above; 7) DO-(CF₂CF₂O)r-D', wherein D and D', equal or different, can be -CF₃, -C₂F₅ and r is an integer such that the viscosity of the product is below the value indicated above.